How to expose a coworker that is outsourcing his work?

One reason you might not want someone outsourcing their work is because they (presumably) won't be able to explain or improve it. Consider testing that? For example, get them on a video call to answer questions about a code review, or do collaborative design work related to their project. You can apply similar practices to what an interviewer would, asking progressively more detailed questions to find out if they really did the work, although you obviously want to present yourself as an interested coworker rather than an interviewer.

At some point, getting your manager and/or their manager involved will be inevitable, and this is exactly the sort of problem that managers and HR exist to deal with. (assuming you're not their manager yourself).

Are you sure they don't just have different Git emails set across their machines or projects?

> How would you go about exposing this? Ideally in a brilliant and clever way.

I wouldn't. If it causes problems, let your employer address them.

Ask for code reviews where they explain the code line by line and overall approach.

Ask for detailed estimates and breakdown on a problem they never heard before, and insist on an initial approach to the solutoin, then verify with what is delivered.

When you do either of these, make sure there are experienced technical people asking questions. Anyone who is technical will not mind explaining code or solutions on the spot. At the very least there should be open conversations on approach, and usually you won't be able to hold that conversation if you are not doing the job.

I'm curious - what evidence is there that made you believe that they "seem" to be outsourcing their work?

- Apply Hanlon's razor. Maybe they're going through some stuff.

- Consider how you'd like to be treated; like for real. What if it was you?

- Burning bridges over something like this isn't worth it. Burning bridges over anything, is almost always a bad idea. Seek to mend.

Don't.